I run exim on Debian stable. I use greylistd for greylisting and dovecot for imap. I backup emails daily by taking a btrfs snapshot and rsyncing it to my home server, which I have a regular backup cycle for. If you were really worried you could setup exim to forward every email to a backup server.

Failover is manual. I would create a new vm, install debian stable and run a script I've prepared a script that installs packages, copies config files and creates users. Downtime with email is not a big deal, servers will retrying sending for a few days. If you have users complaining to you it's obviously a bigger deal.